Simple comments spam solution

If you don’t want people filling website field, simply remove it from the form. Put this code in functions.php of your current theme:

function wpse_remove_comment_url($fields) { 
    unset($fields['url']);
    return $fields;
}
add_filter('comment_form_default_fields', 'wpse_remove_comment_url');

It is more logical than showing the field and advising users against using it.

UPDATE

To not confuse users, and fool some bots, let’s make url field invisible to users, and readable to bots. Add this to style.css:

p.comment-form-url { display: none }

Check url field on submission, if it is not empty, you have possible spam comment. Why possible? Because, today’s, more sophisticated bots, can scan CSS, and JS scripts, to avoid traps. At least, this is a starting point.